The present invention relates to a thin-film forming apparatus utilizing laser beam and more aparticularly to a thin-film forming apparatus capable of forming thin diamond films, cubic system boron nitride and the like having thermal conductivity, electrical insulation properties and a high degree of purity at a high rate over the surfaces of substrates which are substantially maintained at room temperature as an average temperature while controlling the qualities of thin films being formed
In order to carry out the thin film forming techniques in practice, it has been desired that a thin film forming process be carried out at low temperatures so that the adverse thermal effects on the substrates can be avoided and concurrently that the thin film formation can be carried out at a high growth or deposition rate in order to reduce the thin-film formation costs.
In order to attain the above-mentioned conditions, there has been proposed a novel thin-film forming technique called a laser CVD process in which thin-film-forming gases are optically dissociated by high-energy photons released from a ultraviolet laser beam. According to this process, gases can be dissociated only by the high-energy photons so that the process has a feature that the reaction products can be deposited on the substrate at a high deposition rate at low temperatures. However, it has been difficult to form high-quality thin films in a stable manner, since there is not provided a mechanism for controlling the composition of the reaction products resulting from the reaction between the photons and the thin-film-forming gases or for controlling the process for forming a thin film of a reaction product on a substrate.
FIG. 1 is a sectional view of a conventional laser beam CVD apparatus utilizing a laser beam which is disclosed, for instance, in Applied Physics Letter, Vol 43, No. 5, pp 454-456.
In FIG. 1, reference numeral 1 represents an ultraviolet laser beam oscillator; 2, an ultraviolet laser beam; 3, a cylindrical telescope for attaining an energy density of the laser beam required for dissociation of thin-film-forming gases; 4, a window adapted to isolate the thin-film-forming gas atmosphere from the surrounding atmosphere and to introduce the ultraviolet laser beam 2 into a reaction chamber 5; 51, an inlet port for introducing the thin-film-forming gas mixture into the reaction chamber 5; 52, an output port for discharging the thin-film-forming gas mixture to the surrounding atmosphere; 6, a substrate; 7, a suceptor incorporating therein a heater for heating the substrate 6.
The ultraviolet laser beam 2 emitted from the ultraviolet laser oscillator 1 is concentrated by the cylindrical telescope 3 so as to have an energy density sufficient to cause the dissociation of the thin-film-forming gases and then is introduced through the window 4 into the reaction chamber 5 having the thin-film-forming gas atmosphere. The ultraviolet laser beam 2 passes in parallel with the substrate 6 along the path spaced apart therefrom by a few millimeters, thereby causing the dissociation of the thin-film-forming gases. The reaction product is deposited over the surface of the substrate 6 by diffusion.
However, there is a problem in that a thin film formed by the reaction product resulting from the dissociation of the thin-film-forming gases by the high-energy photons includes impurities depending upon the kinds of thin-film-formation gases in that a desired crystal structure may not be obtained as expected. In view of the above, the conventional CVD apparatus shown in FIG. 1 employs a process wherein the surface of the substrate is heated to temperature of hundreds of degrees (.degree. C) by the heater incorporated in the suceptor, thereby controlling the quality of a thin film being deposited on the substrate.
As described above, it has been difficult to obtain thin films having desired properties or quality by the conventional laser beam thin-film forming apparatus, since they have no mechanism for controlling the reaction products. In addition, in order to control the quality of a thin film, only the process of heating the surface of the substrate to hundreds of degrees (.degree. C) has been used in the conventional apparatus In other words, it is impossible to control the quality of the thin films at low temperatures.